national service vs service

national service and service both are nouns.

Word	Noun	Adjective	Verb	Adverb
national service	Yes	No	No	No
service	Yes	No	Yes	No

As nouns, service is a hypernym of national service; that is, service is a word with a broader meaning than national service:

national service (noun)	service (noun)
compulsory service in the military during peacetime	an act of help or assistance; something (such as a tool, software or system) used to render said help or assistance
	the act of public worship following prescribed rules
	(sports) a stroke that puts the ball in play
	the act of mating by male animals
	the act of delivering a writ or summons upon someone
	periodic maintenance on a car or machine
	the performance of duties by a waiter or servant
	employment in or work for another
	work done by one person or group that benefits another
	(law) the acts performed by an English feudal tenant for the benefit of his lord which formed the consideration for the property granted to him
	tableware consisting of a complete set of articles (silver or dishware) for use at table
	a means of serving
	a force that is a branch of the armed forces
	a company or agency that performs a public service; subject to government regulation